The invention relates to a hydraulic drive control in e.g. construction machines.
EP 2123 947 A1 discloses a travel control system for a construction machine with a hydraulic open centre hydraulic travel system, where the displacement of a variable-displacement travel motor is controlled hydraulically. During downhill travelling, the displacement of the travel motor can be increased to prevent acceleration. The hydraulic pressure generated by a proportional valve is overdriven by the pressure in the travel motor, i.e. if a certain pressure value in the travel motor is reached the travel motor is swiveled to the maximum displacement independent from the control pressure given over the proportional valve to the travel motor. Depending on an actual driving situation the displacement of the motor and the pump are changed simultaneously.
It is desirable to provide a method to overcome the problem with speed regulating when the work machine is travelling at top speed and starts to accelerate in a downhill drive and to allow for a comfortable and fuel efficient handling of the work machine.
It is also desirable-to provide a travel system which overcomes the problem with speed regulating when the work machine is travelling at top speed and starts to accelerate in a downhill drive and to allow for a comfortable and fuel efficient handling of the work machine.
A method is proposed for operating a hydraulic driven work machine comprising a hydraulic travel motor having a variable displacement, wherein a flow of hydraulic fluid generated by a hydraulic pump is provided to the travel motor and wherein a control valve is provided for controlling the flow of hydraulic fluid from the hydraulic pump to the travel motor in an open loop, closed centre load sensing hydraulic system. The hydraulic pump hydraulically controlled depending on a load signal generated by the travel control valve and the travel motor is electrically controlled according to a desired driving behaviour of the work machine.
Preferably, the work machine is a wheel driven machine, such as an excavator or the like. Generally, the invention is suitable for all machines which have an open loop travel system. With the proposed invention the displacement of the travel motor can be controlled continuously between minimum and maximum values. The behaviour of the machine can be smoothed so that abrupt stops or accelerations can be avoided.
On the other hand, when the machine is going downhill and the actual speed is exceeding an allowed speed, the displacement of the hydraulic motor can be increased thus slowing down the machine until the allowed speed is reached again. The displacement can be adjusted independent of a travel high pressure in the travel motor which is normally the parameter which determines the displacement of the travel motor. The travel motor can be controlled safely at any time. The displacement of the motor is preferably controlled electrically in a direct way by an electric valve integrated in a travel device which includes the travel motor. Further, the travel motor is controlled between the minimum and the maximum displacement continuously and not stepwise, thus allowing for a smooth behaviour of the travel system. Furthermore the stroke of the spool of the travel section in the main control valve (MCV) may be controlled.
Expediently, the displacement of the travel motor can be controlled proportionally between a minimum and a maximum displacement and moreover, and as a result, the travel speed of the machine can be controlled. The travel motor can be swiveled to any needed displacement and not only to fixed displacements.
The displacement of the hydraulic pump may preferably be controlled in a fuel save mode on travel mode. In the fuel saving mode, flow of hydraulic fluid and displacement are adjusted optimally according to a desired driving behaviour, e.g. according to a set speed.
Favourably, a control unit can calculate how the travel motor displacement, a spool stroke in the travel control valve and the displacement of the hydraulic pump have to be changed to get a desired behaviour, e.g. optimized in view of fuel efficiency and/or handling comfort. In addition to the over speed prevention the invention allows for a jerk free reversing between forward and backward travelling, faster acceleration, smoother accelerating and decelerating, and a fuel saving mode.
According to a favourable embodiment of the invention, the displacement of the travel motor may be adjusted independent of an actual travel high pressure in the travel motor by an electrical signal from a travel motor displacement control valve. The pressure in the travel motor does not override a control signal sent to the travel motor.
In conventional systems, the displacement of the travel motor is just depending in the high pressure in the travel motor. Without any pressure (i.e. at stand still), the travel motor is at minimum displacement in such systems. If the machine starts to travel, the pressure will rise and if it is exceeding a certain value, the travel motor is swiveling to its maximum displacement independent of a control pressure provided by a proportional valve to the travel motor. If the pressure falls below this certain value, the travel motor is swiveled to minimum displacement, according to the conventional motor control, which may result in a nervous, abrupt change of movement of the machine.
Advantageously in the present invention, additional parameters besides the travel high pressure can be taken into account for controlling and adjusting the displacement of the travel motor. Expediently, the displacement of the travel motor may be controlled according to a desired behaviour of the travel motor. The displacement of the travel motor may be advantageously changed to a displacement in the range from maximum displacement to minimum displacement depending at least on a travel high pressure and a travel speed of the machine, for instance.
According to another favourable embodiment of the invention, the flow of hydraulic fluid in at least one pilot pressure line of the travel control valve is generated selectively by different devices depending on a selected control state of the work machine. For instance, a pilot pressure can be generated by the travel actuator or by an independent travel flow valve controlled by the control unit which pilot pressure generating devices can be selected as required.
According to another favourable embodiment of the invention, an electrically controlled valve may select between the flows of hydraulic fluid generated by the different pilot-pressure generating devices depending on a selected control state of the work machine.
The electrically controlled valve may be a solenoid valve which may switch between the different sources of pilot pressure. Particularly, the flow of hydraulic fluid in the at least one pilot pressure line of the travel control valve in a first control state may be generated by a travel actuator and in a second control state may be generated by an independent travel flow valve. Advantageously, the electrically controlled valve (e.g. a solenoid valve) interrupts the connection between the travel actuator and the travel control valve while an independent travel flow valve is connected to the travel control valve instead. A drive control valve can be arranged upstream of the travel control valve and the connection between the travel actuator and the travel direction valve may be interrupted, thus interrupting the connection between the travel actuator and the travel control valve.
In the operational travel mode the machine may be for instance driving downhill, accelerating, driving on flat ground, driving, uphill.
There are two different operational modes which are of interest: a work mode and a travel mode of the work machine. In the work mode the electrically controlled valve for switching between the travel actuator and the independent travel flow valve is activated and switches to the independent travel flow valve so that the signal from the independent travel flow valve is sent to the travel section of the main control valve. In the travel mode the electrically controlled valve is activated to have a smoother starting/stopping/reversing behaviour. If the work machine is driving with constant speed it can be selected either to get the pilot pressure from the travel actuator or from the independent travel flow valve. In this case it has only to be ensured that the maximum control (pilot) pressure (e.g. in the range of 35 bar) is provided to the travel spool to have the maximum stroke of the spool.
In both modes, e.g. the work mode and the travel mode, the signal to the pilot pressure lines can be generated either from the travel actuator (in a control state called first control state) or the independent travel flow valve, i.e. the independent travel flow valve (in a control state called second control state).
According, to another favourable embodiment of the invention, the displacement of the travel motor may be adjustable continuously to displacements between a maximum displacement and a minimum displacement proportional to an electric current control signal of a travel motor displacement control valve. Preferably, the travel motor is set to its maximum displacement without any current on the travel motor displacement control valve, particularly at stand still of the machine.
According to another favourable embodiment of the invention, during downhill driving the displacement of the travel motor may be set to a larger displacement when the machine exceeds its maximum allowed speed. The displacement is increased from minimum displacement to a bigger displacement depending on the decline. During uphill driving the travel motor may be shifted to a bigger displacement when a defined, value for a travel high pressure is exceeded. When the machine is going downhill and the speed is just exceeding the allowed speed, the travel motor is swiveled to a bigger displacement to slow down the machine until the allowed, machine speed is reached again and thus over speeding is avoided. The displacement of the travel motor is increased directly if the machine speed exceeds the allowed speed. Control of the travel motor is fast and reliable.
To improve the acceleration of the machine especially from standstill the displacement of the travel motor may be controlled depending on the high pressure in the travel motor and the travel speed. A displacement curve can be selected in the control unit which improves the acceleration. The displacement of the travel motor can follow the selected displacement curve by expedient parameters sent to the respective components by the control unit, e.g. sent to the independent travel flow valve which generates an optimized pilot pressure for the travel control valve instead of the travel actuator.
By reducing the flow of hydraulic fluid as well as the hydraulic losses, the fuel efficiency of the machine may be improved while travelling and working. The losses are reduced by reducing the flow of hydraulic, fluid. At the same step the pressure will rise because the needed power stays the same. One option is to reduce the speed of the prime mover, i.e. the diesel engine speed, to save fuel directly via the prime mover. The second option (which can be combined with the first option) is to reduce hydraulic losses in the travel system as mentioned above. This can be done mainly by reducing the flow of hydraulic fluid to the travel motor and/or by reducing the flow of hydraulic fluid on the other side, i.e. the hydraulic, pump side. The flow is reduced either by the pump directly in the travel mode or by the travel section of the main control valve in combination with the independent travel flow valve in the work mode. The reduction of the flow of hydraulic fluid may be done either by reducing the stroke of a spool in the travel system in the main control valve (the travel control valve is a part of the main control valve) or by reducing the flow of hydraulic, fluid provided by the hydraulic pump.
According to another favourable embodiment of the invention, in a travel mode of the machine a speed of the prime mover may be set to a predetermined fuel economic speed and/or that the pressure in the travel motor may be increased by reducing the flow of hydraulic fluid from the hydraulic pump.
According to another favourable embodiment of the invention, a pressure difference between a pump side and a load side of the control valve may be reduced for reducing the flow of hydraulic fluid applied, to the travel motor.
Hydraulic losses can be reduced in the hydraulic system.
Additionally or alternatively, for reducing a flow of hydraulic fluid to the travel motor, a pilot pressure in the travel pilot lines can be reduced by establishing a desired pilot pressure with an alternative device to the travel actuator, e.g. by a independent travel flow valve. Hydraulic losses can be reduced in the hydraulic system.
Additionally or alternatively, for reducing hydraulic compensation losses in the travel control valve during work mode of the machine, a travel high pressure may be set to an average pressure value corresponding to the average pressure level in high pressure components in the hydraulic, system by reducing the flow of hydraulic fluid going to the travel motor. Hydraulic compensation losses in the hydraulic system can be reduced.
According to another favourable embodiment of the invention, in a work mode of the machine the displacement of the travel motor may be decreased parallel to the reduction of the flow of hydraulic fluid to the travel motor. When the flow of hydraulic fluid to the travel motor is reduced, the displacement of the travel motor may be decreased at the same time to keep the machine speed.
According to a further aspect of the invention, a travel system for operating a work machine is proposed, comprising a hydraulic pump driven by a prime mover, and a hydraulic travel motor having a variable displacement and being drivable by a hydraulic fluid supplied from the hydraulic pump, having an open loop, closed centre load sending hydraulic system. The travel motor is provided with a travel motor control valve which is controlled electrically and the hydraulic pump is provided with a hydraulic pump controller controlling the hydraulic pump depending on a load signal generated by the travel control valve. Advantageously, the proposed system is an open loop, closed centre load sensing system with flow sharing. “Flow sharing” means that the flow is shared between all active functions but only if the flow which is needed is bigger than the maximum pump flow.
According to another favourable embodiment of the invention, a valve may be provided for connecting operationally the control valve alternately either to a pilot-pressure generating travel actuator or to a pilot-pressure generating independent travel flow valve. This allows to disconnect the travel actuator from the travel control valve.
The independent travel flow valve controls the travel section of the maw control valve. The travel motor is controlled by the travel motor displacement control valve so that the displacement of the travel motor can be changed to any desired displacement.
According to a favourable embodiment of the invention, the hydraulic pump controller may include a pressure reduction valve. The pressure reduction valve allows for reducing the flow of hydraulic fluid thus reducing hydraulic losses in the travel system.
Further, a computer program comprising a computer program code adapted to perform a method or for use in the inventive method described above when said program is run on a programmable microcomputer. Particularly, the computer program may be adapted to be downloadable to a control unit or one of its components when run on a computer which is connected to the internet.
Still further, a computer program product stored on a computer readable medium is proposed, comprising a program code for use in the inventive method on a computer. The computer program and the computer program product may be implemented in the control unit of the travel system.
Further, a vehicle is proposed, particularly comprising at least one of an electro-hydraulic control system including at least one joystick and/or pedal and/or main control valve communicating with a control unit via electric signals. In general, the inventive method and system can be applied to a machine which has an electro-hydraulic control system where the joysticks and pedals and not only the travel actuator are not hydraulic anymore and give instead an electric signal to the respective control unit. In addition, the main control valve may also be controlled electrically.